Heat wheels are used worldwide in buildings where exhausted stale or contaminated air is being exchanged with outside air. The device transfers heat and humidity between the exhaust and supply air streams by rotating between the two adjacent air streams. The wheel transfers sensible heat energy as it absorbs energy in one air stream and emits it in the other. Latent heat energy can be transferred by using a desiccant. By transferring this energy the wheel reduces the work required by an air conditioning unit, providing the owner with a cost saving. There are companies producing this product in Sweden, Japan, India and the USA.
To minimize cross flow of the higher pressure supply air to the lower pressure exhaust air, seals are used. These comprise seals around the circumference (circumference seal 64) of the wheel and also across the diameter (diameter fixed seal 62), where the air flows are separated as depicted in FIG. 1, which depicts seals around the wheel 14 to stop flow between air streams.
The wheel 14 is fluted so that air may flow through it. In order to prevent carry-over of contaminants from the exhaust air to the supply air, a purge system is normally installed. FIGS. 2 and 3 schematically show a fixed single purge that offers protection from contaminants being carried within the wheel 14 from the exhaust flow to the supply flow. The common solution is to angularly displace one of the radial seals 52 relative to a fixed seal 62 so that there is a controlled area where the higher pressure supply air opposes the lower pressure contaminated exhaust air. This results in the supply air pushing the exhaust air back through the wheel, before that section of the wheel 14 rotates into the supply air stream. A purge system is important because it prevents contaminants being recirculated into the conditioned air. Laboratories have specifically stringent requirements in this area.
A double purge may also be used, where the purge air flow passes through the wheel twice and provides improved scrubbing for contaminants. FIGS. 4 and 5 schematically depict a fixed double purge that provides improved performance as purge air passes through the wheel flutes twice by means of two fixed displaced (spaced-apart) seals 56 and a seal 62 in its original position.
The angular displacement required for the purge system is defined by the rotational speed of the wheel 14 and the speed at which the air travels through the wheel 14. The latter is further defined by the media characteristics and the pressure difference between supply and exhaust air streams. The higher the pressure difference the lower the purge angle can be.
To date, purge systems have been fixed, that is, they do not automatically move. The user is able to adjust the purge angle, but it is impractical to adjust the often bolted arrangement regularly as air flow conditions change. The fixed angle restricts system performance. For safety reasons, the purge angle should be designed for the lower operating pressures. But when air flows are higher, the purge is no longer at optimum position and excessive supply air is allowed to short circuit back into the exhaust air flow without flowing through the building first. This wastes energy and reduces cost savings to the customer.
U.S. Patent Application Publication 2008/0108295 to Fischer et al. discloses one approach to attempt to address this problem by varying the rotational speed of the wheel, slowing the speed as pressure difference reduces. This is not an optimal solution as thermodynamic performance is negatively affected.
What is needed is a product that provides an automatically operating and dynamically moving purge system which adapts to prevailing air flow conditions and provides the optimal solution at all times. By solving this problem using an automatically operating and dynamically moving purge system, the customer will have reduced air conditioning costs because the wheel will transfer heat more effectively under varying conditions and there is less cross flow from supply air to exhaust air, so loads on the air conditioning fans can be reduced.